The behavior of a bimetallic water-soluble (Zn-II/Pt-II) porphyrazine hexacation as ligand of G-quadruplex (G4) structures adopted by a human telomeric DNA sequence has been examined with different spectroscopic techniques. In K+ rich solution the hexacationic Zn-II porphyrazine ligand bearing a peripheral cis-platin-like functionality changes the G-quadruplex conformational equilibrium of the human telomeric sequence 5'-d[AGGG(TTAGGG)(3)]-3' and drives it exclusively toward a very stable parallel G4 form in the complex with 2:1 stoichiometry. An increase of the melting temperature of more than 20 degrees C is observed in this complex compared to the G4 alone. On the contrary ligand binding to G-quadruplex of the same telomeric sequence in Na+ rich solution neither markedly influences the predominant basket conformation nor confers increased thermal stability to the G4 structure.